The handover scheme used in radio LAN communication systems is a scheme in which a STAtion (STA) measures Received Signal Strength Indicators (RSSIs) by searching for its neighbor Access Points (APs) by itself, determines an AP to which it will perform handover, and then performs handover to the AP.
However, the STA performing handover needs to perform a scan operation for all channels because it cannot determine in which channels APs exist. Particularly, in an active scan scheme, if an STA broadcasts a probe request message over a channel and APs receiving the probe request message send a probe response message, then the STA measures RSSIs of the probe response messages received from the APs, and uses them in determining an AP to which it will perform handover.
A waiting time of tens of milliseconds (msec) is needed until an STA receives a probe response message after sending a probe request message in a process of scanning all channels to determine an AP to which it will perform handover as described above. The reason why the STA needs to wait for a waiting time of tens of milliseconds until it receives a probe response message after sending a probe request message in the channel scanning process is because the STA needs to prepare for the possible delay in a probe response message due to the channel contention between an AP and STAs, which is caused by the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) characteristics of the radio LAC communication system.
As described above, the current radio LAN communication system uses the active scan scheme as its handover scheme, causing a time delay due to the channel scanning, and the time delay may cause an increase in the time required to determine handover.
In addition, an STA may suffer from an increase in packet delay time and a decrease in data throughput because the STA cannot exchange data with the AP (i.e., a serving AP), to which it is presently connected, while measuring an RSSI of an AP(s) operating in another channel.
Further, an STA may not determine whether the AP (i.e., a target AP), to which it will perform handover, can accommodate or cover the STA, because the STA performs handover after determining an AP to which it will perform handover. For example, while performing a Voice over Internet Protocol (VoIP) call, an STA may not perform Call Admission Control (CAC) in terms of the VoIP capacity of the target AP.
Moreover, since an STA determines a target AP and then performs handover thereto, its AP may not determine the handover start time of the STA. Therefore, the data may be lost, which was transmitted to the AP, to which the STA was connected before handover, until the STA starts an access to the target AP after performing handover.